The present invention pertains to weatherable building materials prepared from lignocellulosic fibers. More particularly, the subject invention pertains to door systems capable of withstanding conditions to which outside entry doors are exposed.
In former times, entry doors were constructed of seasoned or kiln dried wood. Wooden doors are strong and relatively durable. However, their manufacture is relatively expensive due to numerous manual operations as well as the time required to allow adhesives to harden and/or cure during construction. Moreover, as wood is a natural product and susceptible to wide variation in makeup, grain patterns, density and the like, doors often exhibit warping, twisting, or bowing. Exterior wood products exposed to moisture exhibit a tendency to rot. With respect to insulation value, wood leaves much to be desired. Finally, high quality lumber suitable for manufacturing doors, jambs, thresholds, and other entry door system components, is becoming increasingly scarce and expensive. Finger-jointed door jambs and the like have become common for this reason. However, while such components are suitable for painting, the finger joints make them unsuitable for staining and/or varnishing.
Some of the previously identified drawbacks were overcome by the introduction of metal doors of sandwich construction. Such doors generally have a surrounding frame of wood or compressed wood product, and an insulating foam core onto which are joined metal skins. Such doors are less expensive to manufacture, are better insulators, and are highly rot resistant. Unfortunately, the difference in temperature between the inner and outer skins in cold weather coupled with a relatively high coefficient of thermal expansion of the metal can produce a temporary bowing rendering the door difficult to close. More importantly, such doors are susceptible to denting and rusting. Aesthetics of metal doors, even when painted, are less than desirable.
Interior door systems have been made of a variety of compressed wood products. Such doors often have outer skins of compression molded wood fibers or flours with or without interior foam insulation, and structural members of wood or particle board. Wood fibers, chips, and similar starting materials are inexpensive and easily processed on a large scale. Phenol/formaldehyde resol resins or similar resins of urea/formaldehyde, melamine/formaldehyde, and the like are generally used as binders in these systems. However, such doors are incapable of withstanding exterior conditions, and often are unsuitable even for interior locations where humidity extremes are encountered, i.e. basements, bathrooms, and the like.
Manufacture of weatherable building materials from compression molded wood products has not met with a great deal of success. Such products can absorb large amounts of water, plasticizing the fibers and decreasing strength and modulus. When heated by the sun, the water contained in such materials may cause both the product interior as well as surface layers and/or coatings to blister, peel, or delaminate. Coating such components with water impermeable coatings such as coatings of melamine resins, decreases water penetration. However, water may still enter through coating imperfections, cracks, scratches, or through openings necessary for mounting of hardware, i.e. lock sets, door knobs, deadbolts, hinges, and the like. No commercial products in this category have met the accelerated ageing test as more fully defined hereafter.
A recent high density fiberboard exterior door product is believed to employ three layers of coatings, including: a 12 weight percent phenol/formaldehyde partial resin prepress coating applied at approximately 175xc2x0 C., a melamine-fortified urea/formaldehyde prepress sealer coating, and a post-press primer paint coating on the exterior surface. This door system has had poor acceptance in the market because it has experienced repeated warping due to moisture intrusion and unacceptably large deformations where the sun has disrupted the moisture equilibrium within the exterior skin. The manufacturer of this exterior door advises consumers to avoid use of a storm door, where heat may build-up above 54xc2x0 C., again because of drying effects on the dimensional stability of the product. The manufacturer specifies that up to a year may be needed before a warped door caused by moisture variations can correct itself. Many discerning customers in the building trade have found these deficiencies too severe to warrant use of the doors.
The use of phenol/formaldehyde, urea/formaldehyde and other resins provides a binder system which is adequate for interior products slated for use in dry areas. However, the resins are relatively expensive, and must be used in fairly large quantities when products exhibiting high tensile strength and modulus are desired. Thus, European published patent applications EP-A-0 161 766 and EP-A-0 492 016 as well as related U.S. Pat. No. 5,017,319 to Shen teach the avoidance of synthetic adhesives by digesting lignocellulosic materials to the stage where liberated saccharides and oligosaccharides are produced, a portion of these dehydrating to produce furfural and other condensable products which then may react and crosslink, serving as a binder resin. The digestion proceeds in situ and the resulting partially digested fiber/adhesive mixture molded under heat and pressure, or the resinous adhesive may be isolated and applied to lignocellulosic or other substrates in place of more expensive synthetic adhesives.
While the molded products taught by the various Shen patents and publications may be economically produced, they suffer from a deficiency in physical properties such as tensile strength and modulus. Moreover, although the Shen adhesive is described as being waterproof, the latter is a relative term. While products produced according to Shen are suitable for interior use, these products fail both the accelerated ageing test and thickness swelling test, and thus cannot be made into commercially acceptable weatherable, exterior products.
It would be desirable to provide building materials and door systems capable for use in exterior locations which exhibit only low degrees of swelling when exposed to water; which do not exhibit blistering, peeling, or delamination, yet which can be prepared from inexpensive and easily processed lignocellulosic materials. It would be further desirable to produce entry doors and door system components which exhibit high flexural strength and modulus and further exhibit substantial retention of these properties in accelerated ageing tests. It would be still further desirable to provide door systems which can be aesthetically stained and varnished as well as capable of being painted.
The present invention pertains to weatherable building materials of compression molded lignocellulosic fibers which have been partially digested under conditions such that partial breakdown of hemicellulosic and other components takes place; contacting the fibers with a thermosettable novolac resin, optionally in conjunction with other resins, compression molding in the presence of a methylene source, and applying a prepress and/or post press impregnant which provides a water repellant but water vapor transmissive exterior.
The weatherproof construction materials of the present invention employ a unique combination of binders and coatings to produce building materials which are strong and which maintain their strength and modulus properties under extreme conditions. The subject invention materials exhibit little swelling upon exposure to moisture, even after water immersion. Moreover, the unique ability of the subject invention materials to release moisture without delamination, blistering, or other moisture-induced artifacts, allows use of the subject materials in exposed locations such as exterior doors, door jambs, thresholds, trim, etc., as well as in other weatherable building materials such as siding. A particularly useful application of the subject invention materials is in high quality door skins.
The subject invention weatherable materials are composed of lignocellulosic fibers. Such fibers should not be confused with other compressed and resin binder consolidated products such as flake board, oriented strand board, and the like. The materials are prepared from fibers, the majority of which have aspect ratios of 3-80, preferably 4-70, and most preferably 8-30. Finely ground wood products such as sawdust and the like are suitable for use only as a filler in the manufacture of the subject invention materials. Wood chips, flakes, and scrap, among other materials, may be used, but not in virgin form. Rather, during the treatment processes as hereafter described, these or other sources of lignocellulosic fibers must be digested and refined into fibers. By xe2x80x9crefiningxe2x80x9d is meant the process of preparing fibers from lignocellulosic fiber precursor materials such as flakes and particles, etc. Minor quantities of wood flour, wood dust, or other lignocellulosic fines may be included in the lignocellulosic mix. However, such products should not constitute more than 30% by weight of the total lignocellulosics, preferably less than 15% by weight, and more preferably less than 5% by weight.
Other sources of lignocellulosic fibers are well known to the skilled artisan, and include other fibrous lignocellulosic biomass products such as natural grasses, corn stalks, wheat stalks, cotton, linen, flax, and the like. Preferably, however, the fibers are derived from green or dry wood products, i.e. wood chips, flakes, scrap, etc. Both hardwood and softwood may be used, although hardwood is preferred. As the initial treatment of the lignocellulosic fibers involves high temperature digestion at temperatures exceeding the boiling point of water, it is preferred for reasons of economy in energy usage that the fiber source be substantially dry, i.e. have a moisture content less than 18 weight percent, preferably in the range of 4 to 18 weight percent, and most preferably about 8 to 14 weight percent. However, products containing both less as well as more moisture may be used as well. Moisture content may be adjusted as desired by drying the fiber source in hot air or superheated steam, or the fiber source moisture content may be increased by addition of water.
The fiber source is preferably digested at high temperature so as to break down a considerable portion of hemicellulose and other bioses to form an in situ-generated primary binder. In the preferred process according to the invention, this digestion takes place in the presence of a crosslinkable phenol/formaldehyde resin as hereinafter defined, most preferably a novolac resin. However, in less preferred embodiments, it is possible to add the phenol/formaldehyde resin post-digestion. Post-digestion chemical processes which break down the hemicellulose and other bioses may be used as well. Such post-digestion processes which lead to similar fiber/lignin-derived binders are also xe2x80x9cin situxe2x80x9d generated primary binders as that term is used herein.
The lignocellulosic raw materials preferably are digested with steam at 150xc2x0 C.-225xc2x0 C. The duration of exposure to the steam is inversely related to the steam temperature, but is generally for an exposure of 20-200 sec, preferably 20-120 sec. The steam softens the lignins holding the lignocellulosic fibers together prior to the defibering process, which yields finely divided lignocellulosic fiber. Any suitable defiberizing process may be used, including comminution techniques such as a hammer mill, a Szego mill, a single disk refiner for materials of predominantly green wood or a double disk refiner for predominantly lignocellulosic materials with moisture contents of 4-18 weight percent. These defiberizing processes are exemplary, and not limiting.
Some of the hemicellulosic and lignin derived carbohydrates are cleaved by pre-hydrolysis, acid hydrolysis and autohydrolysis reactions as a consequence of the time, temperature, lignocellulose source, as well as presence or absence of steam. The objective of this process step is to cause scission of some fraction of the high molecular weight hemicellulosic and lignin-derived carbohydrates as a means of solubilizing a fraction of the carbohydrates. Determination of the extent of scission is quantitatively difficult, due to the nature of the analytical chemistry extraction methods as well as the variety of natural products involved. The best gauges of the completion of digestion and refinement are by end use tests and by color change of the lignocellulosic fiber mass from light yellows to golden browns. A dark brown color and a pronounced burnt sugar odor indicates that excessive exposure to heat has occurred. This excessive digestion is not preferred for load bearing products. The resultant fiber should have a definable aspect ratio of 4-70, preferably 8-30 to provide mechanical strength and good properties for air laid web formation. Defiberization of the wood chips yields primarily individual wood fibers, but may also leave a distribution of sizes and aspect ratios including some small bundles of fibers, as well as some wood fines. The digestion to the levels required herein are easily assessed by one skilled in the art of wood fiber processing.
While not wanting to bound any particular theory, it is believed that the results of the initial exposure to heat are two-fold. The carbohydrates partially migrate to the outside surface of an area beyond the cell wall through diffusion. These carbohydrates beyond the cell wall begin to decompose through the various reactions leading to partial carbonization. The partially decomposed material appears to form an in-situ generated moisture barrier in intimate contact with the cell wall. In addition, the solubilized carbohydrates appear to react with the phenolic resins in the presence of steam when these are added at this time.
The exodus of some carbohydrates and the scission of the polymers into lower molecular weight units reduces expansion pressure on the cell wall by a reduction in physical mass as well as a reduction in polymer molar mass, also referred to as chain length, of carbohydrates inside the cell wall. The expansion factor varies linearly with a slope of one to approximately two as a polymer changes coil configuration. The magnitude of the change in expansion forces appears dependent on the degree of polymerization, a ratio of polymer weight-average molecular weight to molar mass of the monomer. It is believed that by causing scission of the carbohydrate molecules into lower molecular weight units, the degree of polymerization is reduced. The effect of decreasing the degree of polymerization is to reduce the expansion forces by a multiple ranging from one to approximately two of the decrease in weight-average molecular weight of the remaining carbohydrates.
For example, if scission split every polymer in half, the degree of polymerization would also be reduced by a factor of two. The expansion forces would, therefore, be reduced to a factor ranging from two to approximately four. The expansion force for a given amount of moisture intrusion would only be a half to a quarter as great as the unscissioned material. The result would be less dimensional change, as equivalently less counteracting forces resisting expansion would be required to re-establish equilibrium in the cell.
The dimensional stability can be further improved by causing the effective destruction and subsequent repolymerization reaction of the hemicellulosic and lignin derived carbohydrates. This happens during significant heat exposure in excess of 225xc2x0 C. This improved dimensional stability is counterbalanced by the reduction in the stiffness factor of the polymer molecule because the cellulosic cell wall begins to deteriorate.
Thus, the elevated temperature digestion process is not merely related to the binding capability of in situ produced, bios-derived resins. It has been found that digestion alone, as taught by EP-A-0 161 766, EP-A-0 492 016 and U.S. Pat. No. 5,017,319, all herein incorporated by reference, is incapable of producing acceptable strength properties. It has been found that the strength properties may be improved significantly by addition of conventional binder resins, for example phenol/formaldehyde resol resins. However, these high synthetic resin content products fail thickness swelling tests and, in addition, may exhibit an increased propensity to delamination and blistering. Thus, while high temperature digestion is a necessary prerequisite to preparation of the subject invention materials, it is not alone sufficient, but must be used in conjunction with other binders. However, the simple addition of traditional phenol/formaldehyde binders alone is incapable of producing a truly weatherable product. Both secondary and tertiary xe2x80x9cbindersxe2x80x9d must be added.
The secondary binder resin may be added prior to or during fiber digestion, or may be added after digestion, for example after refinement and collection of fibers. An aqueous or non-aqueous solution or dispersion of phenol/formaldehyde resin, optionally in conjunction with other resins, may be sprayed onto the fibers as they are collected, following collection, or at other times. Resins may also be added in rotary drum mixers, and the like. The exact mode of addition is not critical, although mixing should be as uniform as possible.
Preferably, the phenol/formaldehyde resin is added prior to or during the digestion process. The particular resin should be one which does not cure to a thermoset state under the digestion conditions. For this reason, certain phenol/formaldehyde resins are more suitable than others. The most suitable resins are those prepared at low formaldehyde to phenol ratios. Most preferably, novolac resins are employed. The novolac resins and other resins prepared at low formaldehyde/phenol ratios are somewhat hydrophobic, remain relatively thermoplastic during processing, and may be crosslinked at a later stage to a thermoset polymer.
The amount of secondary binder resin added depends upon the time of its addition, i.e. prior to or during digestion, or post digestion. In general, higher amounts of resin are required post digestion. The amount of binder resin also depends upon the binder type. Relatively larger amounts of high formaldehyde/phenol ratio (resol) resins are required as compared with novolac and low formaldehyde/phenol ratio resins. Addition of epoxy resins and other curable or co-curable resins may decrease the amount of phenol/formaldehyde binder. For example, solid epoxy resins may be added during digestion or just prior to or after refinement, while liquid epoxy resins or those with low melting points must generally be added after fiber collection.
Examples of other resins which may be added in conjunction with the second binder include unsaturated polyester resins as well as thermoplastic polymers such as polystyrene, polypropylene, polyethylene, polyalkyleneterephthalates, etc. The thermoplastic resins may aid in rendering the products more moisture resistant. However, in certain cases, they may lower the physical properties by interfering with the consolidation of the fibers. The suitability of a particular reactive or non-reactive resin additive to be used in conjunction with the phenol/formaldehyde resin may be assessed by conventional physical testing and by the accelerated weathering test. Thermoplastic polymers are preferably added the form of a dispersion or dissolved in solvent. Films of thermoplastic should generally be avoided unless manufactured to allow passage of moisture.
The amount of phenol/formaldehyde resin is broadly between 2 weight percent and 35 weight percent, preferably between 4 weight percent and 30 weight percent, more preferably between 4 weight percent and 25 weight percent, and most preferably between 4 weight percent and 20 weight percent. Amounts of from 8 weight percent to 16 weight percent, particularly 12-15 weight percent are particularly useful. Higher amounts can, of course be used, particularly if added post-digestion, but demonstrate no increase in physical properties as well as potentially lowering desirable properties such as stainability.
In the preferred embodiment, about 10-18%, most preferably about 15%, of a normally solid novolac resin is added to the lignocellulosic material to be processed, and the digestion and refining are carried out in the presence of the novolac resin. It is not known for certain why this process thus far appears to produce products with the greatest moisture resistance and weatherability. Without wishing to be bound to any particular theory, it is believed that the novolac resin may partially react with either the constituents of the cell walls of the fibers or decomposition products of the various bioses and other molecules. By whatever mechanism is operable, the net result is that weatherability and other physical properties are achieved to a greater degree when the phenol/formaldehyde resin is present during digestion as opposed to first digesting and refining, and mixing the same quantity of resin with the previously refined and digested fibers.
The combination of digestion of the lignocellulosic fiber source, coupled with pre- or post-treatment with phenol/formaldehyde resin, creates materials with superior physical and physcio-chemical properties. However, in general, these advantageous properties are still not sufficient for producing building products capable of withstanding outdoor exposure, as reflected by the accelerated weathering test. In order to be suitable for such uses, a tertiary prepress or post press water repellant binder must be applied to the relevant product.
Unlike prior art materials where water impervious coatings, i.e. melamine/formaldehyde resins, have been applied, the tertiary water repellant binders of the subject invention must be water vapor permeable. In prior art coatings, it has been desired to block all access to water. While this total repellency has been useful in minimizing water access, in the real world, scratches, holes for fasteners, screws, nails, hinges, locksets, etc., allow entry of water into the product interior. Prior art coatings are effective or nearly as effective in preventing water egress as water ingress. Thus, water from the interior, when heated, for example by the sun, causes the fibers and binder resins to partially plasticize, weakening both the interior fiber to fiber bonding as well as the adherence of the surface coating. The trapped water is believed to be the major source of delamination and blistering which is often characteristic of prior exterior lignocellulosic fiber products.
It has now been surprisingly discovered that the aim of prior art products and processes: total water impermeability, serves to create or magnify the very problems water impermeability was sought to solve, i.e. freedom from blistering, delamination, etc. The subject invention solves these problems by applying a tertiary binder which is water repellant, but not impermeable, such that moisture which finds itself trapped within a product prepared by use of the subject materials may escape by transmission as a vapor through the water repellant coating. Thus, the tertiary coating of the subject invention materials is broadly characterized as a system which is repellant to water in the sense that the surface will not appear to be wet by liquid water, yet water vapor will readily pass through the coating. On a vertical surface, for example, water will generally run off the surface rapidly, while on a horizontal or near horizontal surface, water will often be seen to xe2x80x9cbeadxe2x80x9d.
It must be emphasized that the tertiary binder is not a simple paint or surface coating, but rather an impregnant which both coats and penetrates to some depth in the product. Traditional factory applied primers and paints, which largely reside on the surface of the building material, are not tertiary binders as that term is used herein, although such coatings may additionally assist in protecting the product. However, it is now clear, that such film-type coatings should be vapor permeable themselves.
The tertiary binder must be highly water repellant, but must also exhibit very high water vapor permeability. Preferably, the water vapor permeability at the surface of the weatherable building material is but little different from that of the essentially non-water repellant substrate prior to application of the tertiary binder. The water vapor permeability may be measured by many techniques. A suitable technique is that of DIN 52615 where the binder to be tested is coated onto Schott glass frits with a porosity of 4, and the permeability expressed as the thickness of a stationary diffusion equivalent air layer thickness, xe2x80x9csdxe2x80x9d. The sd of an untreated frit is approximately 0.03 m. Preferred tertiary binders result in sd values less than 0.3 m, preferably less than 0.2 m, more preferably less than 0.1 m, and most preferably less than about 0.08 m. At the same time, the tertiary binder, to be effectively water repellant, must decrease the wicking action of water along and through the consolidated fibers.
The tertiary binder may be a single component or may comprise two or more components applied simultaneously or subsequently. For example, one component may be applied whose function is to eliminate water wicking while a second component may establish repellency. Both components together, however, must retain water permeability. Preferably, the tertiary binder is a single composition applied prepress or postpress. Prepress application is preferred, particularly when the tertiary binder is heat curable.
Suitable tertiary binders are those which are capable of decreasing the wicking action of water by rendering the wood fibers and other ingredients hydrophobic without totally blocking the pores, etc. The hydrophobic surface coating prevents water from wicking up the fibers, and renders the surface water repellant. At the same time, however, since the pores remain open and hygroscopic pathways (i.e. transport along contiguous oligosaccharides, lignins, etc.) are not blocked, water vapor permeability remains high. It is most desirable that the tertiary binder be substantially non-fugitive; in other words being essentially irreversibly bound to the fiber-containing substrate, whether by strong physical attraction/association or by chemical bonding. Thus, simple non-functional waxes are generally unacceptable unless used in small amounts with other tertiary binders. Waxes chemically treated to contain functional groups which render them non-fugitive are suitable, particularly when used in conjunction with other tertiary binders. The tertiary binder is preferably film-forming, i.e. is capable of forming a film following evaporation of solvent, in the case of soluble binders, or by evaporation of continuous phase in the case of emulsions and/or dispersions. When dispersed in water, microemulsions are preferred over emulsions or dispersions. Suitable tertiary binders exhibit a high water contact angle when deposited as a film on a smooth substrate such as glass or polished metal, but retain substantial water vapor permeability as measured by the glass frit test, or by actual measurement of a treated substrate.
One suitable tertiary binder is VP2100, a microemulsifiable organopolysiloxane containing long chain alkylalkoxysilanes and long chain oligomeric alkylalkoxysiloxanes available from Wacker Silicones. Other suitable polysiloxanes from the same source are Wacker 43A alkoxyalkyl polysiloxane and Wacker BS 1306 amino-functional polysiloxane. Other organopolysiloxanes and organosiloxanes which are emulsifiable in water or solvents, or soluble in organic solvents are suitable, provided water vapor permeability of the substrate is maintained. VP2100 and like microemulsifiable silicones are particularly suitable as they readily form clear to hazy microemulsions upon contact with water; contain reactive groups which render the binder non-fugitive following application; and are highly hydrophobic and thus water repellant. Organosilane and organopolysiloxane compositions containing reactive groups such as Si-H, alkoxy, acetoxy, allyloxy, alkenyloxy, vinyloxy, aminoalkyl, and the like are most suitable. In some cases, addition of a catalyst to the silane and/or siloxane may be necessary, or a short bake at elevated temperature may be required to render the binder non-fugitive. Non-fugitive in this sense means that the binder will not evaporate substantially under expected conditions of use. For example, more than 50% of the applied tertiary binder solids should remain in and on the substrate when treated at a temperature of 70xc2x0 C. for eight hours at 0.1 atmosphere, more preferably greater than 80%, and most preferably greater than 95%, these percents being percents by weight based on binder solids applied. Examples of other silanes and siloxanes which are suitable include those containing haloalkyl groups in addition to or in lieu of alkyl groups, e.g. fluorinated siloxanes or siloxanes substituted with trifluoroalkyl groups.
Other polymers and polymer dispersions are also suitable for use as tertiary binders. Once again, these must be relatively hydrophobic in order to provide the necessary water repellency, and must in general also contain reactive groups or highly polar groups so as to render the binder non-fugitive. Examples are polymers and oligomers of fluoroalkylacrylates and methacrylates and the like, polymeric halovinyl and vinlyidene compounds, especially those polymerized so as to include reactive groups such as anhydride, acrylate, methacrylate, and the like. Once again, the tertiary binder should form a film which exhibits a high contact angle with water and which is water vapor permeable when applied to the substrate. As indicated previously, tests of candidate compositions matching these requirements are easily performed. Fluorinated polymers which have useful water repellant properties are well known, for example, and are available from the 3M Company and DuPont.
The water repellant but water transmissive binders previously described may also be used to advantage with traditional pressed wood products such as those of consolidated wood fibers, flakes, strands, and the like, where the consolidating resin is a phenol/formaldehyde, furfural/formaldehyde, or similar resin. Such products, while not containing primary, secondary and tertiary binders, are nevertheless a considerable improvement over otherwise similar products containing water impermeable coatings.
By xe2x80x9cphenol/formaldehydexe2x80x9d resin as that term is used herein is meant a reaction product of a phenolic compound and an aldehyde, operationally in the presence of other comonomers and nitrogen scavengers such as urea, melamine, dicyandiamide, and the like. Suitable phenols include, for example, 4,4xe2x80x2-dihydroxybiphenyl, resorcinol, and phenol, particularly the latter. Suitable aldehydes are well known and include butyraldehyde, acetaldehyde, and in particular, formaldehyde. For the greatest weatherability, it has been found preferable thus far to use phenol/formaldehyde resins prepared at low formaldehyde/phenol ratios, in particular, novolac resins. A variety of formaldehyde-reactive comonomers may be used in conjunction with phenols, such as the various furans, furfural, melamine, and the like.
Whichever resins are used, it is desirable that suitable curing/cross-liking agents be present. The most preferred agents are methylene sources such as hexamethylenetetramine (xe2x80x9chexaxe2x80x9d). The latter is most preferably used in at least xe2x80x9cstoichiometric amountxe2x80x9d. This amount is about 8 to 12 weight percent based on the weight of phenol/formaldehyde resin solids. Preferably, the hexamethylenetetramine is used in excess, for about 20-30% excess. It has been found that excess xe2x80x9chexaxe2x80x9d is surprisingly efficient in increasing weatherability. However, hexa in amounts in excess of 30% of stoichiometry does not significantly further improve properties, and may cause some properties to decline. An upper limit of hexamethylenetetramine is about 40% in excess of stoichiometry, i.e., about 18% based on the weight of phenol/formaldehyde resin solids. Other methylene sources include paraformaldehyde/ammonium carbonate, and reaction products of aldehydes with aromatic amines. Hexa is preferred.
Numerous additional resins may be added prepress to either enhance physical properties or weatherability, or both. Examples include thermoplastic novolac phenol/formaldehyde, thermoset resol phenol/formaldehyde, naphthylol/formaldehyde, melamine-fortified urea/formaldehyde, resorcinol/formaldehyde, urea/formaldehyde, phenol/furfural, lignin furfural/aniline, and isocyanate resins, to name but a few.
Up to 70 volume percent fillers, additional reinforcing fibers, and modifiers may be added to the material during the prepress blending operations to improve the pressure formed article. These include, but are not limited to thermomechanical pulp, chemithermomechanical pulp, prepared agricultural lignocellulosic wastes, sawdust, excelsior, mineral fillers, micas, wollastonite, glass reinforcing fibers, glass fiber reinforcing veil mats, carbon reinforcing fibers, aramid reinforcing fibers, foaming/blowing agents, fungicides, mildewcides, pigments, dyestuffs, thermoplastic resins, fragrances, and combinations thereof.
Prior to pressure forming, a prepress sealer may be applied to form an additional moisture barrier. The prepress sealer may come in the form of a liquid coating, powder coat, in-mold injected coating, or a film placed in the mold with the material charge. The prepress sealer, like the tertiary binder, must not render the surface water impermeable.
The subject invention building materials, whether door skins, detail moldings, door jambs and/or thresholds, siding, sheetboard, or the like, are molded in a press under heat and pressure, in batch or continuous molding processes. The materials may have a textured exterior dictated by the mold surface. A variety of molds are suitable. Most preferred are molds prepared by nickel coating a cast of a real object whose surface is to be mimicked by chemical vapor deposition, as disclosed in U.S. Pat. No. 5,169,549 which is herein incorporated by reference.
The material is pressure formed at pressure of 120-14,500 KPa with or without steam. Pressure forming processes can include high pressure compression molding, low pressure compression molding, ram extrusion, ram injection, ram injection-compression, hydroforming, explosive forming, twin sheet or single sheet thermoforming with compression assist, vacuum-draw forming, and vacuum-stretch forming, preferably high pressure compression molding between 1,700 kPa and 6,800 kPa.
The steam exposure can range from 0 to 240 seconds, preferably 20-90 seconds at 198-225xc2x0 C., and may be administered in multiple increments once the mold is essentially closed. Venting the mold through releasing mold pressure, evacuating the mold, or opening a stopcock to vent may be needed at several intervals during the pressure forming cycle to prevent blisters and eliminate accumulated volatiles.
The material may be pressed to a thickness of 0.75 mm to 45 mm if steam is applied only from one side of the platen. The thickness may be extended to 100 mm if steam is applied from both sides of the platen. The resultant density of product ranges from 560 kg/m3 to 1200 kg/m3, preferably 800-1000 kg/m3 to eliminate porosity suitable for capillary infiltration, provide mechanical strength, while fostering lower material usage costs as well as reducing the incidence of surface blisters.
In order to reduce the pressure forming cycle time, a post-press infrared, preferably far-infrared; radio frequency; or microwave bake oven may be substituted to continue the curing of the various resins. The curing oven ranges in temperature from 198xc2x0 C.-225xc2x0 C. with exposure inversely related from 20-120 sec depending on thickness of the pressure formed part and the wavelength of energy chosen.
The materials of the subject invention are characterized by an at least 50% retention of initial tensile strength and flexural strength as determined by the ageing cycles specified by ASTM D-1037-95, and a thickness swelling of less than 20%, preferably less than 12%, and most preferably less than 10%. No thickness swelling is, of course most desirable. Thickness swelling in the range of 2-8% is considered exceptional. In addition to the above, the materials of the subject invention must pass the accelerated ageing test defined hereafter.
The accelerated ageing test for exterior door slabs is used as a discriminating test for durability of exterior doors and other weatherable products. Typical environmental cycling completely immerses the sample in the test environment. A more realistic approach to door slab and other exterior product testing exposes only the exterior face of the product to the environmental extremes. The test method described in this standard is applied to door slabs mounted in an environmental chamber such that the exterior face (as installed in the field) of the slab is toward the interior of the chamber. The interior face (as installed in the field) of the slab is toward the exterior of the chamber.
The environmental cycle of the accelerated ageing test makes use of temperature extremes from 104xc2x0 C. to xe2x88x9229xc2x0 C., while also simulating rain and dry conditions. The time for one complete cycle is 24 hours. The test uses extremes of temperature and moisture to accelerate changes in the door slab which occur naturally during exposure to changing weather conditions. Doors are mounted into jambs installed into the walls of the chamber. The installation of the slab exposes individual areas of the door slab or other product to a degree of exposure similar to that they would be expected to receive in a field installation. The chamber is equipped with atomizing spray heads which are capable of completely wetting the exposed surface of any door slab under testing. The chamber is also capable of maintaining the test conditions as described below.
The door slab is representative of standard production and is finished according to the manufacturer""s recommended instructions for residential exterior use. The finish is then given a minimum of 10 days to dry/cure prior to testing. The door slab can be fitted with standard hardware (hinges, lockset) for exterior residential use. The sample is inspected prior to testing. Any warp or cosmetic defects are recorded and any obvious structural defects or deviations from standard product are noted. The environmental cycle is as follows:
A door which passes the accelerated ageing test will have no recorded aesthetic or performance defects after 30 cycles.
The weatherable building products prepared in accordance with the subject invention have a density in excess of 35 lbs/ft3, (560 Kg/m3) preferably in excess of 44 lbs/ft3 (700 Kg/m3), and most preferably about 48 lbs/ft3 (770 Kg/m3) or more; a flex strength in excess of 4000 psi (27.5 MPa); a modulus of elasticity in excess of 3 GPa, preferably in excess of 4 GPa; a thickness swelling of less than 12%, preferably less than 8%, and more preferably less than 5%; and an internal bond in excess of about 1.0 MPa, preferably in excess of 1.5 MPa, and more preferably in excess of 2.0 MPa. Thickness swelling referred to herein is as measured by ASTM D-1037-95.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.